Hydrolysis of vinyl ethers

The observation of a primary solvent deuterium isotope effect (kH/fa>) = 2-4 on the specific acid-catalyzed hydrolysis of vinyl ethers provides evidence for reaction by rate-determining protonation of the alkene.69 Values of kHikD 1 are expected if alkene hydration proceeds by rate-determining addition of solvent to an oxocarbenium ion intermediate, since there is no motion of a solvent hydron at the transition state for this step. However, in the latter case, determination of the solvent isotope effect on the reaction of the fully protonated substrate is complicated by the competing exchange of deuterium from solvent into substrate (see above). 

Kresge and Tobin48 also studied the hydrolysis of vinyl ethers and found a rate ratio of 130 between methyl vinyl ether and ethyl cA-trimethylsilylvinyl ether, corresponding to a stabilization of the /J-silyl carbocation of 2.9 kcal mol-1. In this case the small rate acceleration (compared to the cyclohexyl systems studied by Lambert) can be attributed to the unfavourable dihedral angle. The dihedral angle in the vinyl ether is 90° (24), and on protonation it drops to 60° (25), whereas maximum hyperconjugative interaction requires a dihedral angle of 0°. 

Kresge and Tobin80 investigated the /1-silicon effect on the hydrolysis of vinyl ethers (equation 29) and found a rate acceleration on the hydrolysis of 175 compared with 176, and hence a stabilizing effect of the /1-silyl group on the intermediate -ethoxy carbocation 177 compared with 178. The acceleration is small the rate factor (175) (176) of 129 is equivalent to a free energy of activation difference AAG of 2.9 kcalmol-1,... 

The rapid hydrolysis of vinyl ethers with ethereal perchloric acid, reported87 earlier, was successfully employed28 to transform the enol derivatives 150a and 150b into 3,4 5,6-di-0-cyclohexylidene-2-deoxy-aldehydo-D-xylo-(166a.) and -L-arafoino-hexose (166b), respectively,... 

The mechanism of hydrolysis of vinyl ethers (28) resembles the mechanism of hydration of olefins and has been studied extensively to obtain kinetic results for simple proton transfer to carbon [49]. As in many of the examples previously discussed in this section, the overall reaction is conveniently followed spectrophotometrically and the measured second-order rate coefficient refers to the rate coefficient for proton transfer to olefinic carbon (feH A ). 

The expected change in Bronsted exponent with change in reactivity is illustrated by the results [49] shown in Table 9 for the hydrolysis of vinyl ethers (mono alkoxy-activated olefins) which occurs by initial slow protonation of olefinic carbon as in mechanism (28). The value of R which is the catalytic coefficient for an acid of pK 4.0 calculated from results for carboxylic acids with pK around 4.0 is taken as a measure of the reactivity of the system. The correlation of a with reactivity is scattered but the trend is in the expected direction. The results are quite similar to those shown for the ionization of ketones in Table 2. For the proton transfers shown in Table 9 the Bronsted exponent has not reached the limiting value of zero or unity even when reaction in one direction is very strongly thermodynamically favourable. The rate coefficient in the favourable direction is probably well below the diffusion limit, although this cannot be checked for the vinyl ethers. Non-limiting values for the Bronsted exponent have also been measured in the hydrolysis of other vinyl ethers [176]. 

Variation of Bronsted exponents with reactivity for the acid catalysed hydrolysis of vinyl ethers in aqueous solution (Reprinted with permission from A. J. Kresge et al., J. Am. Chem. Soc., 93 (1971) 413. Copyright by The American Chemical Society.)... 

Since 0i refers to a proton in flight , which is loosely bound, it will be considerably less than unity, leading to k - /kP- > 1, although the factor P (with / = 0.69) will produce a solvent isotope effect which is smaller than that commonly found for pure primary effects. This is in fact what is found for a number of reactions in which the rate-determining step is believed to involve proton-transfer from hydronium ion to carbon, typical values being 1.7 for the reaction of the enolate ion of 2-acetylcyclohexanone with hydronium ion, 1.7-3.0 for the hydrolysis of vinyl ethers by strong aqueous acids,1.7-3.2 for the acid-catalysed cleavage of alkyl-mercuric iodides, and 1.7-2.5 for the hydrolysis of a number of secondary diazo-ketones. " ... 

A good example of behaviour of this kind is provided by the ionization of carbonyl compounds (equation 6), to which additional data for proton transfer from nitroalkanes to various bases may be added [13, 14]. The isotope effect on these reactions rises to a maximum of 10 at AG° = 0, just where the Bronsted exponent is one-half, and it falls off to considerably lower values on either side of this point. The endothermic side of AG° = 0 is particularly well documented here kyjk-o drops to about 3 when AG° 25 kcal mole" and the Bronsted exponent becomes ca. 0.9. Aromatic hydrogen exchange shows a similar correspondence between isotope effect [15] and Bronsted exponent [16], and additional examples may be found in the hydrolysis of vinyl ethers [17] and diazocompounds [18], as well as in the diazo-coupling reaction [19]. 

Reactions of Vinyl Ethers. Vinyl ethers undergo the typical reactions of activated carbon—carbon double bonds. A key reaction of VEs is acid-catalyzed hydrolysis to the corresponding alcohol and acetaldehyde, ie, addition of water followed by decomposition of the hemiacetal. Eor example, for MVE, the reaction is... 

Acidic hydrolysis of l-(<3-methoxyphenyl)pentafluoropropene gives o-hy-droxy-2,3,3,3-tetrafluoropropiophenone by hydrolysis of vinylic fluonne and cleavage of the ether to the phenol [4] (equation 4). 

The mechanism of enamine hydrolysis is thus similar to that of vinyl ether hydrolysis (10-6). 

The metalation of vinyl ethers, the reaction of a-lithiated vinyl ethers obtained thereby with electrophiles and the subsequent hydrolysis represent a simple and efficient method for carbonyl umpolung. Thus, lithiated methyl vinyl ether 56 and ethyl vinyl ether 54, available by deprotonation with t- or n-butyllithium, readily react with aldehydes, ketones and alkyl halides. When the enol ether moiety of the adducts formed in this way is submitted to an acid hydrolysis, methyl ketones are obtained as shown in equations 72 and 73 . Thus, the lithiated ethers 56 and 54 function as an acetaldehyde d synthon 177. The reactivity of a-metalated vinyl ethers has been reviewed recently . 

Starch can be vinylated with acetylene in the presence of potassium hydroxide in an aqueous tetrahydrofuran medium.1 1 The mechanism possibly involves the addition of the potassio derivative of starch across the carbon-carbon triple bond of acetylene, with subsequent hydrolysis of the organometallic intermediate to give the vinyl ether. Such a mechanism has been postulated for the formation of vinyl ethers from monohydric alcohols and acetylene, in the presence of an alkali metal base as catalyst.1 2 The vinylation of amylose is very similar to the vinylation of amylopectin, except for the relative ratio of mono- to di-substitution. With amylopectin, the proportion of disubstitution is greater. In both starches, the hydroxyl group on C-2 is slightly more reactive than the hydroxyl group on C-6 there is little substitution at the hydroxyl group on C-3. 

Studies of vinyl ether hydrolysis have demonstrated a strong retardation effect of jS-carboxy and /J-carbomethoxy groups (2000- to 25,000-fold). The rate profile for (Z)-/5-met boxy methacry lie acid indicates that ionization of the carboxylate raises the rate of hydrolysis by a factor of 240. It has been proposed that this difference in reactivity of ionized and non-ionized forms of the substrate is due to the conjugative and inductive effect of the substituents, rather than /5-lactone formation65,66. 

Reactions of rhodium(III) porphyrins with olefins and acetylenes - Ogoshi et al. [326] have described the reactions of vinyl ether with rhodium (III) porphyrins which are depicted in reaction sequence (33). Step (a) appears to be an insertion of the olefin into the Rh-Cl bond followed by alcoholysis of a chlorosemiacetal to the acetal, step (b) is the hydrolysis of the acetal to the aldehyde. The insertion is thought to start by heterolysis of the Rh-Cl bond producing a cationic species which forms a 7i-complex with the electron-rich olefin. 

Shostakovskil has published research on the synthesis and utilization of vinyl alkyl ethers. Hydrolysis of these ethers to acetaldehyde is probably commercially used today in the U.S.S.R. (290). Other practical utilizations of this class of compounds have been indicated (106,107,108,-176,185,207,283,368). Shostakovskil found that a variety of acid... 

The macromonomer method (C) has also been adopted in cationic polymerization. For instance, amphiphilic graft polymers of vinyl ethers are synthesized by the cationic polymerization of a vinyl ether-capped macromonomer (26) with a block copolymer chain consisting of IBVE and AcOVE segments, followed by alkaline hydrolysis of the latter part into the HOVE units [165], This graft polymer also undergoes a host-guest interaction similar to those with amphiphilic star block copolymers [220]. 

Hydrolysis of 2-alkoxy-3,4-dihydro-l,2-pytans with dilute hydrochloric acid furnishes a convenient synthesis of glutaraldehyde (R = H) and other l,5 dicarbonyl compounds. The starting materials are obtained by the 1,4-addition of vinyl ethers to a,/3-unsaturated carbonyl compounds. The wide selection of diene systems includes acrolein, crotonaldehyde, meth-acrolein, cinnamaldehyde, /3-furylacrolein, methyl vinyl ketone, benzal-acetone, and benzalacetophenone. Ethyl vinyl ether is preferred as the dienophile. The yields in the cyclization step are in the range of 25 87% and in the subsequent hydrolysis Step, 55 85%. ... 

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